The present invention relates to a thermal printing apparatus, and more particularly, to a thermal printing apparatus which controls the thermal energy of a thermal print head (TPH) depending on the kind of printing paper and film employed in a sublimation type thermal printer.
In general, a sublimation type printer, which performs printing by employing a TPH, prints a desired image or picture on a print medium, such as paper, by sublimating the dyes deposited on a film, using the thermal energy emitted by the TPH to which a current is applied.
As shown in FIG. 1, in the above-described common thermal printing apparatus, an analog image signal delivered from signal input sources, for example, a video camera or television, is input as red (R), green (G) and blue (B) signals and converted into a digital signal form in an A/D converter 10.
A first selector 20 selects a signal output from A/D converter 10 or a digital image signal delivered via protocols such as GP-IB, SCSI and CENTRONICS from a digital signal input source, for example, a personal computer or graphics computer.
The signal selected by first selector 20 is stored in a memory 30 in frame units or field units under the control of a memory controller 40, which controls time for writing-in and reading-out of data.
In a second selector 50 constituted by a multiplexer, R,G and B data stored in memory 30 are selectively read-out and provided to a color converter 60. Each of the R, B, and G signals selected by second selector 50 is converted into a complementary color signal. That is, the B signal is converted into a yellow (Y) signal, the G signal is converted into a magenta (M) signal and the R signal is converted into a cyan (C) signal in color converter 60.
In corrector 70, various corrections, for example, gamma, color, resistance and temperature corrections are performed on the output of color converter 60 and the result is written into a line memory 80 in line units.
Gradations of the data read from line memory 80 in line units are compared with a predetermined gradation value in a middle gradation converter 90. Then, a Strobe signal, which indicates a heating time period in the units of the compared gradation value, is generated, and a TPH 100 is heated for the heating time, to thereby perform a color printing operation. Thus color printing is out by printing each of Y,M and C colors on a single piece of recording paper.
This can be explained as follows. When data is read from memory 30, data of a single vertical line is read out by second selector 50 with respect to an initial B signal and converted into a Y signal via color converter 60 and written into line memory 80. A middle gradation conversion is performed upon the data written into line memory 80 in middle gradation converter 90, and the converted data is delivered to TPH 100 to complete one line printing. In a video printer which performs printing on A6 sized printing paper (hereinafter, the printer will be called an A6 thermal printer), printing the Y color, i.e., the complementary color of B color, is completed when approximately 500 to 600 lines are printed for a single screen.
Then, in second selector 50, data corresponding to the amount of one screen is applied to line memory 80 in units of one vertical line with respect to the G signal. Thus, printing the M color, i.e., the complementary color of G color, is completed through the above mentioned procedure. Then, the R signal for a single screen is selected by second selector 50 and the R signal for a single vertical line is read out from line memory 80. Thus, printing the C color, i.e., the complementary color of R color, is completed through the above-mentioned procedure.
The A/D converter 10, 1st selector 20, memory 30 and memory controller 40 constitute an image signal processing circuit 1. The second selector 50, color converter 60, corrector 70, line memory 80, middle gradation converter 90, and TPH 100 constitutes a print control circuit 2. An image display circuit which enables an output of image signal processing circuit 1 to be displayed onto a display device, such as a video monitor, may be provided as part of the present invention. In addition, a system controller, not shown in the drawings, for controlling the entire system can be added.
FIG. 2 is a block diagram of the middle gradation converter 90 and the TPH 100 shown in FIG. 1.
Referring to FIG. 2, an address generator 91 generates a write address and a read address for line memory 80. The write address is provided to line memory 80 so that the data corresponding to a single vertical line can be written into line memory 80. When an end address of the write address is generated, an end pulse is generated by address generator 91, thereby enabling a gradation counter 92. The gradation counter 92 operates when data is read out from line memory 80 by the read address generated by address generator 91. Thus, when the data corresponding to a single vertical line has been written into line memory 80, the gradation counter 92 is enabled and a read address is output simultaneously by the address generator 91 in order to make line memory 80 perform a reading operation.
In gradation counter 92, data for gradation level 1 in the form "0000 0001" is output to an erasable programmed ROM (EPROM) 93 and a gradation comparator 96. In gradation comparator 96, a "high" signal is output when the gradation level of the printing data output from line memory 80 is higher than a gradation level output from gradation counter 92, while a "low" signal is output when the gradation level of the printing data output from line memory 80 is lower than a gradation level output from gradation counter 92. The comparison result is delivered in turn to a shift register 101. For example, in an A6 thermal printer, approximately 512 printing data are shifted to shift register 101 and stored therein. This example is for the case of A6 sized paper where the number of heating elements of TPH 100 for printing one line is 512.
If the gradation comparing signal is "high", gradation level 1 can be printed as follows.
Here, if the optical density of gradation level 1 corresponding to the data "0000 0001", is assumed as 0.2, procedures for printing with an energy E1 onto a print film as shown in FIG. 3 are as follows. As shown in FIG. 4, a strobe signal relevant to a time period t1 which corresponds to energy E1 is generated in time width generator 95 and applied to a latch register 102. Then, a heating element 103 is heated for the duration of time t1 which corresponds to energy E1 so as to express gradation level 1.
To perform a heating for gradation level 2, gradation counter 92 outputs the data "0000 0010". Then, in gradation comparator 96, the data read from line memory 80 and the output data of gradation counter 92 are compared and the above-described operation is repeated.
Here, the heating time corresponding to gradation data generated by gradation counter 92 is preprogrammed in EPROM 93. Then, an electronic switch 94 is operated according to an output of EPROM 93 and a Strobe signal is generated in time width generator 95 in units of each gradation level so as to be output to latch register 102.
When the output of shift register 101 is latched to latch register 102, the latched output heats the TPH 103 for a duration t2 corresponding to the width of the pulse generated in time width generator 95.
Thus, when 256 gradations finish heating according to the above-described procedures, which means the completion of one line printing and 500-600 line heatings for one screen in an A6 thermal printer is also completed. Y, M and C color heating is performed in the same manner as the above, to thereby perform a color printing.
In this case, a plot of heating energy with respect to an optical density of each gradation is shaped as an S-curve, that is, heating energy is proportional to the optical density as shown in FIG. 3. The optical density increases as the heating time is lengthened as shown in FIG. 4.
In FIG. 3, curve (1) shows the sensitivity of a plain printing paper while curve (2) shows the sensitivity of an overhead projector (OHP) transparent film, both of which are known print medium for a thermal printer. Since each type of print media has different print sensitivity, the printing method can be varied depending on the printing media.
Both plain paper and OHP film can be used in a conventional thermal printer. The heating value of each heating element within the TPH should be lowered when a picture is to be printed on OHP film than in the case where a picture is printed on plain paper. OHP film has a lower thermal conductivity and a higher surface smoothness as compared with plain paper. That is, since plain printing paper has a better sensitivity than OHP film, OHP film print density is remarkably reduced when printing is performed under the same conditions as for plain paper.
Conventional techniques for solving the above problems are shown in FIG. 5 to FIG. 7.
FIG. 5 is a schematic block diagram showing a thermal printer where the supply voltage for the TPH is controlled. The device shown in FIG. 5 comprises a key controller 110 for setting printing mode, a general purpose computer 120, i.e., an input signal source for providing a picture to be printed, and a first sensor 150 for sensing the kind of printing media supplied to a paper supply mechanism (not shown) of the thermal printer.
A micro-computer 130 is connected to the output terminals of key controller 110, general purpose computer 120 and first sensor 150 in order to control the printing operation according to a predetermined printing mode. The micro-computer can be a system controller. A power supply 140 is connected to an output terminal of micro-computer 130 to provide operating power for TPH 100. The TPH 100, as in FIG. 2, includes a shift register, latch register and heating resistances to sublimate the ink deposited on a sublimation film, is connected to an output terminal of power supply 140.
The device shown in FIG. 5 operates as follows to print on an OHP film. A command for printing on an OHP film is input to micro-computer 130 from key controller 110 or from general purpose computer 120. When OHP film is inserted in the supply mechanism, such as a supply tray, the first sensor 150 recognizes the OHP film by sensing the different reflection or transmission rates of light for plain paper and OHP film. The sensor 150 informs the micro-computer 130 of the insertion of the OHP film. In micro-computer 130, a control signal is generated and supplied to the power supply 140 to cause the power supply 140 to supply to the TPH 100 a voltage that is increased for the OHP film printing mode as compared to that supplied for a normal printing mode where plain paper is used.
In power supply 140, the output voltage varies according to the control signal supplied from micro-computer 130. In TPH 100, the heating energy varies in response to the output voltage supplied from power supply 140. That is, if the printing media is OHP film rather than plain paper, more heating energy is provided for printing.
In general, the heating energy (E) can be expressed as follows. EQU E=(V.sup.2 /R).times.t (1)
where (V) is the voltage applied to TPH, (R) is the resistance of the heating element, and (t) is the heating time. As shown in expression (1), heating energy (E) is proportional to voltage (V). Thus, the fact that the print density and the image quality of OHP is lower than that for plain paper can be taken into account by controlling the voltage in accordance with the printing media being used.
FIG. 6 is a schematic block diagram showing a thermal printer which controls TPH heating time to compensate for the differences in printing modes. Components of the printer of FIG. 6 which are the same as those of FIG. 5 are denoted by the same reference numerals, and explanation thereof will be omitted.
An middle gradation controller 90, as in FIG. 2, includes an EPROM 93 wherein heating time by gradation units in a normal printing mode and in an OHP printing mode is programmed, is connected to an output terminal of micro-computer 130. In addition, TPH 100 is connected to an output terminal of middle gradation controller 90.
The device shown in FIG. 6 operates as follows. A user selects an OHP printing mode through key controller 110 or computer 120 and the mode selection is recognized by micro-computer 130. When a sheet of print media is inserted into the print media supply tray, the first sensor 150 senses if the inserted media is plain paper or OHP film. If the sheet is OHP film, micro-computer 130 recognizes that the OHP printing mode for printing a picture on the OHP film is the correct printing mode. Micro-computer 130 generates a control signal which is supplied to a higher address port of EPROM 93. The EPROM 93 outputs heating time data in accordance with the control signal input to the higher address port. Here, heating time data for a plain paper printing mode and heating time data for an OHP film printing mode are programmed in EPROM 93. For example, in the case of expressing the same gradations, if heating data "0000 0010" is stored in EPROM 93 for a plain paper printing mode, then, heating data "0000 0100" is stored in EPROM 93 for an OHP film printing mode.
Accordingly, in an OHP film printing mode, the pulse width of the strobe signal, which indicates the heating time, is wider than in a normal printing mode. The strobe signal (STB) is applied to TPH 100 in response to the output data of EPROM 93 of middle gradation converter 90. As a result, more energy is provided for an OHP film printing mode than for a normal printing mode.
In addition, if general purpose computer 120 is connected online to micro-computer 130, the digital signal applied via general purpose computer 120 is printed after performing the above-described procedures according to the printing mode.
Another conventional thermal printer, which varies a strobe signal according to the type of print media, is disclosed in U.S. Pat. No. 4,795,999. The printer disclosed therein automatically selects a heating time having the optimum heating value for each heating element according to the print media used, i.e., plain printing paper or OHP film.
FIG. 7 is a schematic block diagram showing a thermal printer which varies the heating energy by varying the repetition of print cycles in dependence upon the print mode and the print media used. Components of the printer of FIG. 7 which are the same as those of FIG. 5 are denoted by the same reference numerals, and explanation thereof will be omitted. The device shown in FIG. 7 operates as follows. If an OHP film printing mode is selected by key controller 110 or general purpose computer 120, micro-computer 130 controls a print mechanism portion 200 to perform the printing procedure for a normal printing mode twice in response to the above selection.
When an OHP film is inserted, first sensor 150 senses the presence of the OHP film and sends a signal to the micro computer 130 indicating this presence. Here, print mechanism portion 200 comprises the TPH 100 and directly performs a printing operation. In this case, unlike the case of FIG. 6, the effect of actually extending the heating time (t) is obtained by repeating the printing procedure.
Further, in an OHP film printing mode, the heating energy can be enlarged by raising the voltage supplied from power supply 140 as in FIG. 5 and by simultaneously controlling the heating time as in FIG. 6. In addition, a sublimation film exclusively used for OHP film and exhibiting a high degree of heating under the same applied heating energy can be used.
However, if the appropriate sublimation film for a given print media and a predetermined printing mode is used, this cannot be recognized by the conventional systems. As a result, the predetermined printing mode is performed as it is, and a print having a low density and low screen quality is obtained, which deviates from the user's desire. Therefore, re-printing has to be performed.
A printer disclosed in U.S. Pat. No. 4,795,999 has different printing methods for each printing mode by varying the heating time depending on the printing media. However, a printing method which automatically varies depending on the kind of the dye-deposited sublimation film is unavailable.